Abstract

Polycyclic aromatic hydrocarbons (PAHs) are among the detrimental soil contaminants worldwide. Free microbial cells have been widely used for PAHs biodegradation in contaminated soils. However, only few studies have been carried out on the application of the immobilized cell system for bioremediation of PAH contaminated soil/slurry. In this chapter, we reviewed the literature on PAHs biodegradation, especially in solid/slurry phase by immobilized cell systems. This was followed by our experimental study on bioremediation of a clayey soil contaminated with phenantherene, a model contaminant, at 250, 500, 1000, and 2000 (mg PHE)(kg dry soil)−1 using immobilized cell (IC) and free cell (FC). Formation of intermediate metabolites (IMs) of the PAH oxidation pathways was also examined since some of the IMs could be reluctant to further biodegradation or be more toxic than the parent pollutants. This was performed by measuring the total organic carbon (TOC) of soil and chemical oxygen demand (COD) of aqueous phase. Additionally, the effect of temperature (10, 20, 30, 40 °C), pH (5, 7, 9), and inoculum size (300 and 600 mg/L) on IC and FC systems was investigated. According to t-test results, immobilization had an insignificant effect on PHE biodegradation up to 1000 mg PHE (kg dry soil)−1. However, less accumulation of IMs was observed at higher PHE content of soil for IC as compared to FC system, showing the superiority of immobilization. TOC analysis also showed the oxidation of PHE in soil samples into nontoxic intermediate metabolites that in turn were mineralized by our microbial consortium. Maximum PHE biodegradation was obtained at 30 °C, pH of 7, initial 2000 (mg PHE)(kg dry soil)−1 and 600 mg L−1 of inoculum for both the systems. However, it was shown that the IC system was tolerable to acidic condition with retaining 85% of its biodegradation capability at neutral pH while the FC system was highly affected by acidic pH and retained only 33% of its capability. Our results show the superiority of cell immobilization compared to free cell system, especially at high concentration of PHE and under harsh environmental conditions. This can have generic application for other PAHs.